Transform a list of maps in Elixir - functional-programming

Question

I have a list of maps, that I get from querying my database.It has a datetime field that I want to transform into another list of maps where the datetime field is transformed to it's corresponding epoch value.
What i have:
[
%{
m_id: 267,
end: #DateTime<2020-03-07 17:30:00Z>,
start: #DateTime<2020-03-07 14:30:00Z>,
type: "normal",
s_name: "smum",
w_id: 256
},
%{
m_id: 267,
end: #DateTime<2020-03-07 07:30:00Z>,
start: #DateTime<2020-03-07 04:30:00Z>,
type: "normal",
s_name: "smum",
w_id: 256
}
]
What i want to transform it to:
[
%{
m_id: 267,
end: 12356789, #some epoch value for eg
start: 12367576, #some epoch value for eg
type: "normal",
s_name: "smum",
w_id: 256
},
%{
m_id: 267,
end: 12334567, #some epoch value for eg
start: 12354767, #some epoch value for eg
type: "normal",
s_name: "smum",
w_id: 256
}
]

Answer 1

To transform a single map, you can do
%{map | end: DateTime.to_unix(map.end), start: DateTime.to_unix(map.start) }
So just Enum.map over the list to apply this to all list members:
Enum.map(list, fn map -> %{map | end: DateTime.to_unix(map.end), start: DateTime.to_unix(map.start) } end)
(I suspected there may be a problem using the map update syntax here because end is a reserved word, but I tested in https://www.jdoodle.com/execute-elixir-online/ and it works.)

Answer 2

I would go with Kernel.SpecialForms.for/1 comprehension.
for %{start: s, end: e} = map <- list do
%{map | start: DateTime.to_unix(s), end: DateTime.to_unix(e)}
end
It’s slightly different from Enum.map/2 solution, because it would discard those elements not having either start or end keys. To handle those properly, one should use Map.update/4 wisely.